A dual fuel engine can typically operate in two modes, namely, a dual fuel mode (gaseous fuel and liquid fuel) and a liquid fuel mode. In a strictly liquid fuel mode, a liquid fuel, such as diesel fuel, is injected directly into an engine cylinder or a precombustion chamber as the sole source of fuel and energy during combustion. In a dual fuel mode, a gaseous fuel, such as natural gas, is controllably released into an air intake port of the particular cylinder where the gaseous fuel is mixed with air. After a predetermined period of time, a small amount or pilot amount of diesel fuel is injected directly into the cylinder or precombustion chamber in order to ignite the mixture of air and gaseous fuel. Compression of the air/fuel mixture during the compression stroke ignites the liquid fuel which in turn ignites the air/gaseous fuel mixture.
The amount of minimum liquid pilot fuel necessary for delivery to each individual cylinder to assure good combustion may or may not be the same for each such cylinder due to differences in the operating characteristics of the fuel injectors and valves used to control liquid fuel delivery to such cylinder. Assuming the amount of minimum liquid pilot fuel necessary for delivery to each individual cylinder will be substantially the same for each such cylinder, due to variations in the operating characteristics of the liquid fuel injectors, each liquid fuel injector may require a somewhat different current pulse time in order to achieve the same fuel delivery to each respective cylinder. If the control signals used to deliver fuel to different cylinders are of the same duration, such variations can result in different amounts of liquid fuel being delivered to different cylinders. In addition, because the amount of liquid pilot fuel being delivered to a dual fuel engine is generally small compared to the amount of gaseous fuel being delivered to such engine during a dual fuel operating mode, any variation in the amount of liquid pilot fuel being delivered to the respective cylinders will be magnified and this could seriously effect optimal ignition and combustion of the air/gaseous fuel mixture within each such cylinder.
On the other hand, if the amount of minimum liquid pilot fuel necessary for delivery to each individual cylinder is, in fact, different, the combustion performance of each such cylinder must be evaluated in order to separately determine the least amount of liquid pilot fuel necessary to achieve optimal combustion performance in each respective cylinder. If too little liquid pilot fuel is utilized, optimum ignition and combustion will not be achieved and ignition within any particular cylinder may occur at a retarded timing, or such cylinder may misfire due to incomplete ignition of the gaseous fuel. In either case, deteriorated cylinder performance occurs.
It is therefore desirable to both determine the least amount of liquid pilot fuel necessary to achieve optimal ignition and combustion of the air/gaseous fuel mixture within each particular cylinder of a dual fuel engine for any particular engine operating condition, and thereafter deliver the corresponding minimal amount of liquid pilot fuel to each such cylinder. It is also desirable to minimize fuel cost and reduce the emissions associated with a dual fuel engine.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.